1. Field of the Invention
The present invention relates to an apparatus and a method for manufacturing a plastic container which is provided with high barrier property and returnable property, especially relates to an apparatus and a method for forming a diamond-like carbon (DLC) film on an inner surface of a plastic container so that the film is uniform in its thickness, or an apparatus and a method for mass-producing plastic containers where the DLC film is formed on an inner wall of the container so as to be uniform in its condition in the whole of the container.
2. Description of the Related Art
Generally, a plastic container is used in various fields such as foods, medical supplies or the like, because of its various properties. Namely, for example, it is easy to form and is light in weight. And furthermore, the cost of producing the plastic container is low.
However, it is well known that plastic has undesirable properties of permitting low molecular gas such as oxygen or carbon dioxide to permeate through it, or permitting bringing sorption of low molecular organic compound (a property of absorbing molecular organic compound into a plastic composition). Therefore, the purposes of plastic container or the cases where the container can be used are widely limited in comparison with other container formed of glass or the like.
For instance, when the plastic container is used for holding a beverage which is apt to deteriorate by oxidization, there is some possibility that the beverage in the container deteriorates with age because oxygen can permeate plastic and reach the inside of the container. And when the plastic container is used for holding a carbonated drink such as beer, there is a fault that time in which can be spent on a distribution of the drink unfortunately comes to be shorter as compared with the other containers such as glass containers or the like because there is the possibility that the drink goes flat by releasing of carbon dioxide gas through the plastic wall of the container.
When the plastic container is used for holding a beverage, such as orange juice, including aroma compositions, the balance of the plural aroma compositions is broken and quality of the beverage is spoiled because low molecular organic compounds as the aroma compositions such as limonene included in the orange juice are absorbed in plastic. So, cases where the plastic container can be used as a container for beverages including such an aroma composition is unfortunately restricted.
And, the plastic container may deteriorate purity of material held in the container by elution of low molecular compound, such as plasticizer, residual monomer or other addition agent, included in the plastic compositions. Therefore, the plastic container is not used as a container for holding a substance specially in a case where purity of the substance held within the container should be maintained.
On the other hand, in recent years, making good use of resources is specially asserted and recycling of spent containers is a subject of study at present. However, if the plastic container is used as a returnable container, various low molecular compositions, for example, compositions causing a smell such as an odor of mold, are absorbed into plastic. Since these compositions absorbed into plastic are remained after washing of the plastic container, it is not sanitary to recycle it. There is also the possibility that the low molecular compositions as heterogeneity gradually soaks into contents held in the container to deteriorates the contents.
Therefore, in a case where the plastic container is used as a returnable container, it is necessary to inspect the plastic container which has been collected for the purpose of checking whether there is any residual heterogeneity. So, it is extremely rare to use the plastic container as a returnable container at present because the inspecting operation as to residual heterogeneity is a time-consuming job.
However, since the plastic container has the merits that it is easy to form, is light in weight, and is low-cost, it is very convenient to be able to use the plastic container for holding beverages such as carbonated drinks, beverages including flavor composition or material whose purity has to be maintained. And it is also greatly convenient to be able to use the plastic container as a returnable container.
The applicant of the present invention proposed an invention by which the plastic container can be used as a returnable container in the previous patent application (Japanese Patent Application No. 189224 of 1994), paying attention to convenience of the plastic container.
The invention of the previous application is related to an apparatus for forming DLC (Diamond Like Carbon) film on the inner surface of the plastic container so as to improve the property of gas barrier of the wall and prevent from absorbing of low molecular compound into the plastic container.
The DLC film mentioned above is hard carbon film also called "i carbon" or "amorphous hydrocarbon", and is amorphous carbon film mainly consisting of SP.sup.3 bond. The DLC film, which is extremely hard, has good electrical insulating property and high refractive index.
In the previous application, an apparatus for manufacturing the above plastic container coated with the carbon film is proposed. By the apparatus, the DLC thin film is formed on the inner surface of the plastic container to produce the plastic container which can be used as a returnable container through preventing permeating of gas via plastic and absorbing low molecular organic compound by forming the DLC film.
As shown in FIG. 25, the above apparatus for manufacturing the plastic container coated with the carbon film is provided with ceramic insulating plate 2 attached on a base 1, an outer electrode 3 attached on the insulating plate 2, and an inner electrode 4 inserted into a chamber formed in the outer electrode 3.
The outer electrode 3 composes a vacuum chamber in its inner space. The vacuum chamber is prepared for plasma discharge. By a vacuum pump not shown, the vacuum chamber is exhausted via a exhaust tube 5 until the chamber comes to be evacuated, after a plastic container B is inserted into a body part 3A and the chamber is sealed by a lid part 3B.
After spouting out the raw gas which has been supplied via a providing tube 6 into the chamber of the electrode 3 from spouting holes 4A formed in the inner electrode 4 so that a raw gas is diffused uniformly in the evacuated chamber, electric power is given to the outer electrode 3 by a high frequency power source 8 through a matching circuit 7. In this way, plasma is generated between the outer electrode 3 and the inner electrode 4 connected to ground earth, and then the DLC film is formed on the inner surface of the plastic container B by plasma.
In this apparatus for manufacturing the plastic container coated with the DLC film, the chamber of the outer electrode 3 is formed in a shape almost similar to the outer surface of the plastic container B, and the outer surface of the inner electrode 4 is formed in a shape almost similar to the inner surface of the wall of the plastic container B, therefore, the gap between these electrodes is made uniform. And the DLC film is formed only on the inner surface of the wall of the plastic container B because the raw gas is spouted into the inside of the plastic container B.
Furthermore, in the apparatus, since the outer electrode composes the vacuum chamber, the time required for evacuating the vacuum chamber can be remarkably shorten, and therefore, the mass production of the plastic returnable container comes to be realizable.
As mentioned above, the DLC film is restrictively formed only on the inner surface of the wall of the container B for the purpose of avoiding deteriorating quality of the plastic container B. If the DLC film is also formed on the outer surface of the wall of the plastic container B, there is the possibility that the thin and hard DLC film formed on the outer surface is damaged because the containers collides with each other or are rubbed each other during a process of manufacturing in a factory or a channel of distribution in a case where the plastic container is used as a returnable container.
The plastic container is extremely lighter than usual returnable containers such as a glass container. Therefore, it is feared that the plastic container falls down when they are carried on a conveyor on purpose to fill or cork the same. And, in a beverage factory or the like, it is necessary to fill the container at a high speed in order to obtain high productivity, therefore, the light plastic container has to be supported while the container is being filled with contents in order to prevent the plastic container from falling down.
And furthermore, if the plastic container is not supported during corking the container after the filling process, the shape of the plastic container may be changed by the high pressure in the vertical direction of the container given through the corking process, or the corking may come to be incomplete because the degree of intensity of the plastic container is weaker than that of other containers such as a glass container.
Therefore, as shown in FIG. 26, the plastic container has sometimes a supporting ring Bb positioned below a screw Ba formed in an opening portion of the container. This supporting ring Bb is formed as a projection in the shape of a flange which projects from the peripheral surface of the opening portion of the plastic container. When the plastic container B is carried while the container is filled and corked, the supporting ring Bb slides on a guide rail with the under surface of the supporting ring Bb touching the guide rail. As a result, falling of the plastic container B is prevented during the process of filling or corking, and the load in the vertical direction given during the corking process is received by the guide rail.
However, if the DLC film is formed in such a plastic container B having the supporting ring Bb in the opening portion of the container by the apparatus which is mentioned in the previous application and shown in FIG. 25, a gap around the opening portion of the container is inevitably formed. That is to say, as shown in FIG. 27, when the plastic container B is inserted into the outer electrode 3, no gap is made around a trunk portion and a shoulder portion of the plastic container B. However, a gap between the peripheral surface of the opening portion and the inner surface of the outer electrode 3 is inevitably made around the opening portion of the plastic container B because of forming the supporting ring Bb in the opening portion.
Accordingly, adhesion strength of the DLC film stuck to the inner surface of the plastic container in the area around the opening portion having the supporting ring Bb is necessarily lower. And the DLC film of the plastic container gets spotted because the thickness of the DLC film in the above area gets smaller than that in the area around the trunk portion and the shoulder portion.
There is also the problem that the gap between the inner surface of the outer electrode 3 into which the opening portion B' of the plastic container is inserted and the outer surface of the plastic container comes to be large and the DLC film formed in the plastic container gets spotted because the outside diameter of the screw Ba which is formed in the opening portion B' comes to be larger than the outside diameter of the other areas in the opening portion.
And, in the above apparatus for manufacturing the plastic container coated with the carbon film mentioned in the previous application, the shape of the outline of the inner electrode 4 which is inserted into the plastic container B as the object of coating is restricted by the inside diameter of the opening of the plastic container B. Therefore, in the case where the opening portion is formed so as to be slenderer than other portions, the gap between the outer electrode 3 and the inner electrode 4 in the area from the shoulder portion of the plastic container to the opening portion thereof may come to be smaller than that in the area of the trunk portion of the plastic container.
In consequence, when plasma is generated between the outer electrode 3 and the inner electrode 4 by giving electric power to the outer electrode 3, plasma is concentrated into the area from the shoulder portion of the plastic container to the opening portion thereof where the gap between the outer electrode 3 and the inner electrode 4 is smaller. Therefore, the following phenomena are occurred. For example, spots or patches in the DLC film are made because the thickness of the DLC film formed in the container has necessarily large dispersion and distortion of the opening portion of the plastic container B is caused by heat given through the plasma process.
On the other hand, the plastic container with the DLC film has to be produced in large quantities for distribution on a massive scale when the plastic container is used as a returnable container in a market. Moreover, the conditions of the coated DLC film must be same among all containers that is mass-produced, and manufacturing cost must be low.
To meet the above demands, the inventor of the present invention carried out the following experiment for mass-producing the plastic container coated with the DLC film. As shown in FIG. 28, plural chambers (three chambers Cr, Cc and Cl illustrated in FIG. 28) are arranged. The common high frequency power source Rf is connected to the chambers via a matching box M and the electric power is given to the chambers by the common high frequency power source Rf to try to manufacture the plural plastic containers with the DLC film at the same time.
However, in order to mass-produce the plastic container coated with the DLC film with the number of the chambers being increased as shown in FIG. 28, the conditions have to be kept even among the all chambers Cr, Cc and Cl to make the conditions of the DLC film in the containers completely even as to all the chambers. But the electric power given to the chambers could not actually get even among all chambers in the above experiment. Therefore, irregularity was caused among the DLC films of the plastic containers.
As a result of the experiment, it is recognized that when increasing the number of the chambers to mass-produce the plastic container coated with the DLC film, to keep the coating conditions same among the all chambers is extremely difficult in a technical viewpoint and the most important problem for mass-producing the plastic container coated with the DLC film is to discover the method for keeping the coating conditions for the DLC films even among the all chambers.
And furthermore, in order to lower the manufacturing cost of the plastic container with the DLC film, to shorten a duration for a coating process comes to be a problem.